The present invention relates to a method for production of a rotor for centrifugal compressors.
The term centrifugal compressor normally defines a machine which restores a compressible fluid, at a pressure which is greater than that at which it received the fluid, imparting to the latter the energy necessary for the change of pressure by means of use of one or a plurality of rotors or impellers, each of which consists of a certain number of blades, which are disposed radially such as to form a certain number of passages which converge towards the centre of the rotor.
More particularly, high-pressure centrifugal compressors are normally equipped with rotors 20 which are tested in a standard manner.
It should be noted that, for a given diameter, the geometry of the rotor is fixed, and is the same for different applications, a fact which will assume particular relevance in the following description of the method according to the invention.
More particularly, the main parts of these rotors for centrifugal compressors are the hub, the shroud and the blades.
Owing to the high density of the gases, a truly critical problem in the technical field of high-pressure compressors is that of guaranteeing the stability of the rotor.
Combined forces, which are proportional to the density of the gas, and are induced by the labyrinth for the gases, can cause unacceptable sub-synchronous vibrations which have an effect on the said rotor.
In fact, the sensitivity of the rotor to these forces increases more or less in proportion with the flexibility of the rotor.
In addition, the entire dynamic behaviour of the rotor is particularly critical for this type of application, and the simplest means for increasing the rigidity of the rotor in the same operative conditions (same bearings, same temperature, same pressure of the gas, etc) is that of using shafts which have large diameters.
The diameters both of the shafts and of the hubs have thus been increased in the known art, such as to increase the lack of sensitivity to external disturbances, and therefore to improve the characteristic rotodynamic behaviour of high-pressure centrifugal compressors.
The spacers have thus been eliminated, and the aerodynamic path is obtained directly on the shaft.
More specifically, two rings, the front one of which is in two pieces, and the rear one which is in a single piece, maintain the axial position, whereas a coating with a high level of hardness protects the shaft.
These characteristics assure that the aerodynamic path does not change, and ensure that this configuration is particularly efficient in increasing the rigidity of the shaft; however, these changes mean that a different technology must be taken into consideration for production of these rotors for centrifugal compressors.
With particular reference to the known art, it should be noted that the blades are conventionally obtained by means of milling the hub (or shroud), and subsequently the welding is carried out from the interior of the aperture, in order to join the parts.
Another technique is used for rotors with a low flow coefficient, in which the blades are too thin to make it possible to carry out the welding from the interior.
In fact, in this technique, the two parts are slot-welded from the rear of the hub (the blades are machined on the shroud), owing to the presence of the nose or front portion of the shroud, which has the same diameter as the main end of the blade.
After the welding has been carried out, the heat treatment is also carried out, which, as will be appreciated, reduces the residual tensions which have formed.
Each rotor is separated from the others by a spacer, in order to create the necessary aerodynamic space at the intake of the rotor.
Thus, on these bases, it is apparent that it becomes very difficult to increase the diameter of the hub, and thus the diameter of the slot-welding, from the exterior. BRIEF DESCRIPTION OF THE INVENTION
The object of the present invention is thus to provide a method for production of a rotor for centrifugal compressors, such that the above-described requirements are fulfilled.
Another object of the present invention is to provide a method for production of a rotor for centrifugal compressors, which makes it possible to obtain a very high level of dimensional accuracy, which is in line with the required expectations.
Another object of the present invention is to provide a method for production of a rotor for centrifugal compressors, which provides the possibility of obtaining a high level of structural resistance of the rotor.
A further object of the present invention is to provide a method for production of a rotor for centrifugal compressors, which makes it possible to obtain a high level of dimensional accuracy, as well as an optimum surface quality of the finished product.
These objectives and others according to the invention are achieved by a method for production of a rotor for centrifugal compressors, wherein the said rotor is produced from a monolithic disc, provided with a central hole, characterised in that it consists of use, within an isolating medium, of at least one first electrode which has polarity opposite that of the polarity of the said rotor, wherein the said first electrode operates starting from the outer diameter of the said monolithic disc, in order to produce the blades and the cavities of the said rotor, and wherein this processing takes place with a continuous path consisting of a first step of roughing, followed by a second step of finishing with a tool which has a shape similar to that of the electrode used for the first roughing step, in order to produce an accurate geometry of the said blades.
According to one embodiment, the method according to the invention includes a further step of finishing, in which an electrode operates on the surfaces of the blades, at the inner portion of the said rotor, in order to round the main ends of the said blades, on the inner side of the disc.
As an alternative, according to a preferred variant of the method, an electrode 15 carries out at least the first step of roughing wherein the said electrode is shaped such as to be able to produce the profile of each of the blades, both on the pressure side and on the suction side.
In fact, this electrode has a first and a second portion, which are separated from one another by the space which is necessary for the presence of a blade.
Further characteristics of the method for production of a rotor, according to the present invention, are defined in the other claims attached to the present application.
The characteristics and advantages of the method for production of a rotor for centrifugal compressors, according to the present invention, will become more apparent from the following description of a typical embodiment, provided by way of non-limiting example, with reference to the attached schematic drawings described below.